# Centre of Mass (or Gravity)

Imagine you could squash a pool ball up to the size of a pinhead. An arrow drawn vertically downwards from that point shows the centre of gravity, acting at the centre of mass.

Where might the centre of gravity of a pool cue be? Obviously at a point nearer the handle end where the wood’s diameter is greater.

Finding C of M is easy. Cut out a funny shape from a cereal packet. Make a tiny hole in one corner with a pin, making sure the piece of card can move freely and hang a piece of cotton with a weight on the end on it. Draw a line down the vertical string. Do exactly the same somewhere else on the card. Where the lines cross is the centre of mass. The force of this mass acting downwards acts along the line of the centre of gravity.

Some objects have a centre of mass outside of the material of the object, like  a wooden lab stool. All the mass is in the seat, so the centre of mass is in thin air somewhere underneath the seat.

Where’s the centre of mass of a Bunsen burner? Probably in the metal bit at the base. It’s in stable equlilbrium so if it’s tipped it’ll fall back on to its base again.

Remember the OSMIUM ROCKER from the density post. Think about what would happen. You might need your density table handout. Look back at it and suggest what might happen if you sat on the rocker

Can you balance a potato on a knife edge? Sure you can. Two forks lower the centre of gravity to below the knife edge so that if it’s tilted, it’ll tend to return to the equilibrium position on the knife edge. Try it at home…ASK FIRST.

You might think about this as the ‘tightrope walker principle’. The only reason tightrope walkers don’t fall off is that the massive pole they carry lowers their centre of mass so it’s below the rope. in fact, they CAN’T fall off.

In 1859, a man called Blondin first walked across Niagara Falls on a tightrope. He did it lots of times, once carrying a man on his back! Notice the pole!

London buses can tilt an awfully long way before toppling over also. See a bus doing the tilt test here. Why doesn’t it topple over? Because the centre of mass is low, almost between the wheels, so as long as the centre of gravity acts inside the pivoting wheel, it won’t topple over.

Now, something for you to do. Tails on birds can lower their centre of mass so they can perch on wires. Cut out a parrot like this one from a piece of card about 25-30 cm high. Weight the tail with a heavy clip – it helps if you curl the tail down underneath the bird a bit. See if your parrot can stand on a wire on his own… Colour it in and bring them all to school and we’ll display them.